1. Field of the Invention
This invention is related in general to the field of electrophoretic analysis of liquid samples. In particular, the invention relates to an apparatus and method for completely automating the electrophoresis process beginning with the step of applying liquid samples to an electrophoresis support media and without moving the support media further including the steps of electrophoresing, staining, incubating, drying, scanning, and performing densitometry measurements on the scanned samples.
Electrophoresis is the science of moving charged particles in an electric field through a solid or semi-solid media. The technique is most commonly used in medical research in medical laboratories for analyzing various blood proteins.
2. Description of the Prior Art
In the diagnosing of ailments of human beings and animals, it is known that much information can be provided by an analysis of certain biological fluids such as blood serum proteins, lippo proteins, hemoglobin and isoenzymes. It is well known tha electrophoresis is an effective method of separating the respective components of such fluids for a microscopic analysis or for employing optical densitometry techniques in analyzing the samples.
In the basic method of electrophoresis, charged molecules of the sample fluids are separated under the influence of an electrical field wherein the liquid sample to be examined is applied to a support medium having a buffer moistened porous surface. Because the various components of the fluid move at different rates through the support medium, the liquid sample may be separated into its respective components. Subsequent staining of the fractional components in the support medium may then be subjected to optical densitometry or other methods for examination.
The electrophoresis process has been performed through a series of manual steps for many years. The manual process typically has started with the operator preparing an electrophoresis chamber by filling appropriate cavities of the chamber with buffer solution. Buffer solution is a liquid used in the electrophoresis process to maintain the support medium surface in a moist condition and to provide an electrical interface to a power source applied to the chamber so that an electric field may be applied to the support medium. The support medium is typically a piece of MYLAR (trademark) backing which has been coated with a gel substance such as cellulose acetate or agarose. The liquid sample to be examined is typically a blood serum, but of course may be other liquids, the components of which may be moved through an electric field.
After the operator has prepared the electrophoresis chamber, he then applies as precisely as he can, consistent volumes of the samples to precise locations on the support medium. The operator then places the support medium into the electrophoresis chamber so that the edges of the support medium are immersed in two buffer cavities at each of its longitudinal ends. Electrophoresis is then performed using a precise and consistent high voltage applied for a precise and consistent interval of time across the buffer cavities.
After electrophoresis has been completed, the operator applies a uniform coating of staining reagent or stain to the surface of the support medium allowing a precise and consistent interval of time for the reagent and sample to chemically combine. The staining reagent is a liquid used after electrophoresis to chemically combine with the separated components of the fluid sample, causing its components to exhibit optical characteristics.
Next, the operator places the support medium into a temperature controlled oven and incubates it using a precise and consistent temperature and time interval. Incubation is the process of controlling the chemical reaction between the components of the liquid sample and the staining reagent by means of applying heat for a fixed interval of time.
Next, the operator dries the sample plate by increasing the oven temperature for a second precise and consistent temperature and time interval. The drying process stops the reaction between the sample plate and the reagent by removing water from the support medium.
One of the problems associated with the manual support medium preparation is that the liquid samples to be analyzed are multiply applied to the support medium which is to be subjected to electrophoresis. The samples may be applied to the support medium one at a time in serial fashion with a hand pipettor, but the hand pipettor must be rinsed with a cleansing agent and blotted before a new sample is aspirated and then applied to the strip. Applicators have been designed to apply fluid samples simultaneously or in "parallel" to the strips. Such applicators are described at page 61 of the General Products Catalog for 1984-1985 of Helena Laboratories with offices in Beaumont, Tex. Such applicators may apply eight, twelve or more samples to a microporous support medium and have the advantage of making the electrophoresis technique easier and more reproducible.
Such applicators however have been essentially non-automatic and have required cleaning of the applicator tips after each application to the support medium. A disadvantage of the prior art applicators is that there has been no means for automatically washing and cleaning the barrels of the pipettes during each cycle time so as to prevent contamination of each of the barrels during application of a new plurality of fluid samples to a new support medium. Another disadvantage of the prior art applicators is that there has been no means for precisely automatically applying a very small amount--of the order of one microliter--of sample liquid to a support medium. Another disadvantage of the prior art is that there has been no means for precisely automatically diluting a very small amount of the order on one microliter of sample fluid with a diluting liquid and precisely applying a very small amount of the diluted sample to a support medium.
There have been prior art apparatus and methods available for automatically performing electrophoresis and staining of the plurality of samples applied to a support medium. For example, U.S. Pat. No. 4,360,418 to Golias and U.S. Pat. No. 4,391,689 to Golias describe an automated electrophoresis and staining apparatus and method.
Such apparatus includes an electrophoresis chamber and a series of vats mounted upon a platform and arranged in a row where the vats are adapted to contain respectively a liquid stain and a series of plate processing solutions. The plate holder rack, having a horizontal open frame, supports an upright electrophoresis plate or support medium onto which has been applied a sample for electrophoretic fractionization. Such electrophoresis plate had to have been previously prepared by applying liquid samples either manually or by using one of the parallel applicators described above. The plate is nested within the chamber within an electrophoretic circuit for a predetermined time period. A power operated lift and transfer assembly is provided on the base and is adapted to lift, transfer and lower the plate holder rack and plate from the chamber progressively into each of the underlying vats for a predetermined period in a linear stepping motion maintaining the plate in an upright position at all times. It is noted that the staining process relies on chemical procedures for the staining process rather than the manual system described above where incubation and drying are used. Although the apparatus described above has many desirable features, it has a practical disadvantageous feature in that it requires providing a plurality of chemicals and wash solutions in the unit which must be maintained periodically.
Prior art apparatus and methods for optically scanning support media which have been subjected to electrophoresis and staining have used devices such as photomultiplier tubes, photodiodes or similar devices which produce an electric current or voltage output proportional to the light falling on such device. These devices are generally referred to as detectors. Prior art instruments employing these detectors are used for determining various physical properties of the samples which have been prepared by electrophoresis. The properties of interest concerning the separated bands of the sample are size and optical density or intensity of emitted light which is of a wave length different from that of the excitation light source. Separated bands of each sample which have been subjected to electrophoresis are known components of the sample under test and it is desirous that they be quantified for the purpose of aiding in medical diagnosis or research.
The known instruments which use the detectors referred to above generally find it necessary to use a blocking optical slit. The purpose of the slit is to allow the detector to "instantaneously view" a portion of the sample plate which is the same relative size and shape as the slit. The detector then produces an electrical current or voltage which is proportional in amplitude to the magnitude of the light detected. The current or voltage produced is then converted by means of an analog to digital converter and the resultant digital representation of the light magnitude is stored in an organized format in a digital computer memory.
Although an alternative embodiment of the invention described below uses prior art detectors in combination with other automatic electrophoresis apparatus, a preferred embodiment of the invention includes the use of video electronic scanning of the samples on the support medium which have been prepared by electrophoresis. Video electronic scanning is preferred in recognition of well known problems of using such prior art scanning detectors. One of the problems of using such prior art instruments is that the blocking slit requires a very precise width and length. If the length is too great, some of the detected light may actually be the result from an adjacent sample. If the length is too small, all of the light from the sample currently being scanned may not be detected. With a plurality of samples on a plate it may be necessary to change the physical slit size from sample to sample.
If the slit physical width is too great, it is possible that the light from adjacent bands of the plurality of samples being scanned could be detected causing the boundaries to be difficult, if not impossible, to determine. If the width is too small, it is possible that the detector output will be erratic and not yield correct proportional results.
Another disadvantage of the prior art slit/detector system is that in order that the entire sample be observed, it is necessary that each sample be mechanically scanned by moving either the detector or the sample plate. The movement must be at a very constant speed and free of vibration in order that the digital data being collected by the A to D converter is an accurate representation of both the optical density and physical size of the components of the sample.
In order that a plurality of samples may be scanned, it is necessary that the detector or sample plate be moved in yet another axis such that the scanner may scan a sample and then step over to the next sample and continue the scanning process. The step-over movement must be accurate and repeatable to insure that the detector is truly seeing the entire sample and only the desired sample.